Creating the simplest Addon
I finally understood what this script does! It is not displayed through any button or menu as expected at first. That´s logic because there is no code that refers to any button or menu. As a script it can not do anything either because it only contains the definitions of the functions but there is no call to any function. The only way to visualize its action is through Open the user File -> User Preferences, Select the Addon section. You write in the search box the name of your addon, in this case "My Test Addon". Once you show it, you give it to the square on the right, a checkbox, that action will enable it, if you press the checkbox again, you will disable it. Each time you enable it, it will write "Hello World" in the console (to see it click on Window-> Toggle System Console) and each time you disable it, "Goodbye World" will be written.